Vehicle braking systems



6 Sheets-Shet 1 M. J. ENGLAND ET AL VEHICLE BRAKING SYSTEMS Filed Jan.29, 969

Dec. l, 1?@ J. gmg n ETAL 3,543,512

VEHICLE BRAKING SYSTEMS 6 Sheets-Sheet 2 Filed Jan. 29, 1969 1 m w wmm am @E@@@@% I QR g W a g P Q P @qgga i@ E mm M Dec. 1, mm WM. ENGLAND ETAL543,51

VEHICLE BRAKING SYSTEMS Filed Jan. 29, 1969 6 Sheets-Sheet 3 ET AL3,543,51 6 Sheets-Sheet 4 wm A VEHICLE BRAKING SYSTEMS N A L G N E im MFiled Jan. 29, 1969 mm H Dec 1, 397g g. E GLAND ETAL 3,543,512

VEHICLE BRAKING SYSTEMS Filed Jan. 29. 1969 6 Sheets-Sheet 5 D N A L G NE Iw M VEHICLE BRAKING SYSTEMS 6 Sheets-Sheet 6 Filed Jan. 29, 1969 I II I I I I l I I I I l I l I I I I l l I I I I I I I I I l l l I l l I II I I I I l I I I I I I I I I l I I I I I I I l I I I I I l States US.Cl. 60-51 Claims ABSTRACT OF THE DISCLOSURE A hydraulic accumulator,e.g. for a trailer brake system, is charged from a constant volume pumpintended for operating hydraulic services through open-centre typecontrol valves. An unloader valve connected in series with theopen-centre valves unloads the pump responsively to the accumulatorreaching a predetermined pressure. The accumulator may be a dual purposeaccumlator in that it is adapted to automatically release a spring toapply the trailer brakes mechanically in the event of loss of hydraulicpressure from the accumulator. A metering valve may be provided on thetractor for controllably operating hydraulic actuators for the trailerbrakes utilizing the trailer mounted accumulator or utilizing anotheraccumulator mounted on the tractor. In the former case the tractor andtrailer lines are releasably interconnected by a quick-release,twin-line, self-sealing hydraulic coupling. The accumulator and brakelines on the trailer are automatically interconnected to apply thetrailer brakes hydraulically in the event of trailer breakaway.

This invention relates to hydraulic systems and to vehicle brakingsystems.

According to one aspect of the present invention, a vehicle fitted witha constant volume hydraulic pump for operating hydraulic servicesthrough open centre or open neutral valves is provided with an unloadervalve connected in series with the valves of the hydraulic services andserving to charge a hydraulic accumulator, the unloader valve beingadapted to switch to a pump unloading position after the hydraulicaccumulator has been charged to a predetermined pressure by the pump.

Preferably the vehicle is a tractor vehicle for towing a trailer vehiclefitted with the hydraulic accumulator and with a hydraulic actuator forbrakes of the trailer vehicle, the hydraulic accumulator serving as asource of hydraulic fluid for the hydraulic actuator. A metering valveresponsive to an applied signal is preferably provided on the tractorvehicle for connecting the accumulator to the hydraulic actuator. Whenthe tractor vehicle is fitted with a hydraulic braking system themetering valve can be conveniently made responsive to the pressuredeveloped in the master brake cylinder of the tractor vehicle.

Advantageously, the hydraulic accumulator is adapted to mechanicallyapply brakes of the trailer vehicle (preferably the previously mentionedbrakes) when the accumulator pressure falls below a predetermined value,the accumulator and the actuator having separate lines lead ing torespective coupling heads on the trailer vehicle which are adapted to becoupled to corresponding heads on the tractor vehicle, the meteringvalve being operable to connect the tractor vehicle coupling heads toone another responsively to an applied signal in order to use hydraulicfluid from the accumulator to operate the actuator.

Preferably said trailer vehicle coupling heads are fitted withself-closing valves which automatically close when ice the trailervehicle coupling heads become detached from the tractor vehicle couplingheads and a valve arrangement is provided on the trailer vehicle,preferably adjacent the coupling heads, for interconnecting said lineson the trailer vehicle in the event that the coupling heads becomedetached from one another inadvertently.

A preferred embodiment of accumulator comprises a stressed spring actingmechanically to apply the brakes and a piston and cylinder mechanismadapted by the pressure of pressure medium contained in an accumulatorchamber in the cylinder to further stress the spring for relieving themechanically applied brake force. Advantageously the pressure in theaccumulator chamber acts upon further resilient means, such as a cushionof gas to provide hydraulic energy for normal service braking.

According to another aspect of the present invention, a tractor vehiclefor towing a trailer vehicle fitted with a hydraulic accumulator, ahydraulic actuator for the trailer brakes and respective accumulator andbrake lines to trailer vehicle coupling heads, is provided with ahydraulic fluid supply pump connected via an unloader valve to a firsttractor vehicle coupling head adapted to be coupled to the trailervehicle accumulator line coupling head, the unloader valve being adaptedto switch to a pump unloading position after the hydraulic accumulatorhas been charged to a predetermined pressure by the pump, and a meteringvalve adapted to apply the trailer vehicle brakes responsively to anapplied signal by connecting the first tractor vehicle coupling head toa second tractor vehicle coupling head itself adapted to be coupled tothe trailer vehicle brake line coupling head.

The invention is further described, by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 illustrates that part of a vehicle braking system constructed inaccordance with the invention mounted on a tractor,

FIG. 2 illustrates that part of the braking system of FIG. I mounted ona trailer to be coupled to the tractor,

FIGS. 3 and 4 are views similar to FIGS. 1, and 2 respectively, of asecond embodiment-of vehicle braking system,

FIG. 5 is the circuit of that part of a third embodiment of vehiclebraking system mounted on a tractor and,

FIG. 6 illustrates that part of the braking system of FIG. 5 mounted ona trailer to be coupled to the tractor.

Referring to FIGS. 1 and 2 of the drawings, a vehicle braking system fora trailer receives its supply of hydraulic fluid from a hydraulic pump10 fitted to a tractor to which the trailer is intended to be hitched.The pump 10 serves to provide basic tractor hydraulic services withhydraulic fluid and operates with constant delivery. A single actinghydraulic cylinder 11 is shown in FIG. 1 by Way of example fed from atwo-position control valve 12 which has an open-neutral position, ie inits neutral position it discharges the fluid fed via a line 13 to itsinlet port by the pump 10 to a line 14. In practice a number ofhydraulic services will be provided on the tractor and in most casesthese will comprise double acting devices controlled by three-positionopen-centre valves. The valves are connected in series in such a waythat when one valve in the series is in its open-centre or open-neutralposition it passes the fluid fed from the pump 14 to the next valve inthe series. In other words the line 14 instead of leading away from thehydraulic services as illustrated would be connected to the inlet portof the next valve in the series. Thus when all the valves are in theiropen-centre or open-neutral positions the pump 10 discharges directly tothe line 14 which conducts the discharged fluid for return to areservoir 15, from which the pump draws hydraulic fluid through a line16. When one of the valves, such as the valve 12, is operated, the fluiddischarging through the 3 line 14 is throttled to develop the pressurein the line 13 and the fluid delivered by the pump is delivered at thiscontrolled pressure to the hydraulic cylinder 11. The cylinder 11 isconnected by the valve 12 to a return line 17 leading to the reservoirwhen the valve 12 is in its neutral position illustrated. Thus thehydraulic system for the tractor services is basically a constant volumesystem in which the pressure varies according to the demand by theservices. However to operate the trailer brakes a source of hydraulicfluid under constant pressure is required and this source is obtainedfrom the pump 10 by means of an unloader valve 18 whose inlet port 19 isconnected to the line 14.

The trailer braking system shown in FIG. 2 comprises a hydraulicaccumulator 170 and at least one hydraulic actuator, two such actuators21 and 22 being illustrated by way of example. The pistons 23 of theactuators operate the brake shoes or brake pads, a cam arrangement 24for this purpose being shown by way of example. That part of the brakesystem on the trailer is connected to the part of the system On thetractor by means of a twin, quick-release self-sealing breakawaycoupling 25 (FIG. 1). An accumulator line 26 (FIGS. 1 and 2) leads fromthe trailer coupling head 27 to the accumulator 170 and a brake line 28leads from the trailer coupling head 29 to the hydraulic actuators 21and 22.

The hydraulic accumulator 170 is a dual purpose accumulator in that itnot only provides a source of hydraulic fluid under pressure foroperating the trailer brakes hydraulically but also serves to apply thetrailer bakes mechanically in the event of a loss of pressure from theaccumulator. The accumulator 170 comprises a fixed body 171 in one endof which a piston 172 is slidable. The rod 32 of the piston 172 isconnected by a connecting rod 33 to a yoke 34, the ends of which areconnected by brake wires 35 to the individual pistons 23 of thehydraulic actuators 21 and 22. A spring 36 acts between the body 171 anda sleeve 37 which has at one end a flange abutted by a collar 173 on thepiston 172. Thus the spring 36 urges the piston 172 to the left in adirection to apply the trailer brakes mechanically and is suflicientlyprestressed to effect full application of the trailer brakesmechanically. A handbrake lever 38 is coupled to the connecting rod 33for applying the trailer brakes independently of the spring 36, theconnecting rod 33 having a lost motion connection 39 with the piston rod32 for this purpose. The accumulator body 171 contains a cylindricalcavity 174 which is divided into chambers 175 and 176 by a free piston177. The chamber 175 serves to store the hydraulic fluid under pressureand is connected via a bore 178 in the piston 172 to the line 26. Thechamber 176 is charged through an inlet (not shown) with a gas under asuitable pressure to provide a gas cushion. The gas cushtion can bereplaced by a compression spring acting between the piston 177 and theleft hand end of the body 171.

The accumulator chamber 175 is charged with hydraulic fluid through theunloader valve 18 and a line 44 (FIG. 1) leading from a pressure outletport 45 of the unloader valve 18 to the tractor coupling head 46 coupledto the trailer coupling head 27. The unloader valve has a discharge port47 connected by a line 48 to the reservoir 15. The unloader valvecontains a conical closure mem ber 49 biased by a spring 50 towards avalve seat 51. The closure member 49 is attached to a piston 52 ofslightly larger diameter than the valve seat 51. The valve inlet chamber53 at the left hand side of the piston 52 is connected by a passage 54to the inlet port 19. The inlet port 19 is connected through anon-return valve 55 to the pressure outlet port 45. A pressure chamber56 at the right hand side of the piston 52 is also connected to thepressure port 45. The valve chamber 57, serving as the outlet chamberand accommodating the spring 50, is connected to the discharge port 47.

The unloader valve 18 enables the pump 10, which may for example be agear pump and which normally operates under constant delivery quantitybut variable delivery pressure, to charge the accumulator chamber 175 toa pre-determined pressure.

When the pump 10 is first started up and assuming that none of the basictractor hydraulic services is operating, i.e. all the valves are intheir open-centre or open-neutral position, the fluid delivered by thepump flows through the lines 13 and 14 to the inlet port 19 of the valve18. Initially the spring holds the closure member 49 against its seat 51so that the hydraulic fluid flows past the nonreturn valve to thepressure outlet port 45 and through the accumulator lines 44 and 26 tothe accumulator chamber 175. The accumulator is shown in FIG. 2 in itscharged condition but initially the piston 172 is in its leftwardposition and the piston 177 is in its extreme rightward position inwhich it abuts a stop 179 in the chamber 175. When the force of thepressure acting on the piston 172 becomes as large as the initial forcein the spring 36 the piston 172 is moved to the right therebysimultaneously relieving the trailer brakes of the mechanically appliedforce. When the pressure of the hydraulic fluid exceeds the pressure towhich the gas cushion in the chamber 176 is initially charged the piston177 moves to the left and balances the pressures in the chambers 175 and176.

Whilst the non-return valve 55 is open and the accumulator 170 is beingcharged with hydraulic fluid, the forces of the spring 50 and thepressure in the valve chamber 53 acting over the difference between theareas of the piston 52 and the valve seats 51 hold the closure member 49closed against its seat 51 against the force of the accumulator pressureacting in the chamber 56 upon the piston 52. When the accumulatorpressure reaches a predetermined value determined by the spring 50 andthe pressure in the chamber 53, the closure member 49 is moved to theleft thereby opening the valve to connect the inlet port 19 to thedischarge port 47 which is at reservoir pressure. Once the valve hasbeen cracked open the opening movement is rapid since the pressure inthe valve inlet chamber 53 falls but the non-return valve 55 holds thepressure in the biasing chamber 56 at that of the accumulator. In thefully open position, the pressure in the chamber 56 acting on the piston52 has completely overcome the force of the spring 51 The valve closuremember 49 can only close again when the pressure in the accumulatorchamber 175 has fallen to a lower pressure at which it can no longer actwith sufficient force on the piston 52 to overcome the force of thespring 50. Whilst the closure member 49 is held in its illustrated fullyopen position by the pressure in the accumulator chamber 175 the line 14leading from the tractor hydraulic services is freely connected to thereservoir 15 via the unloader valve 18 and the discharge line 48 so thatthe tractor services can operate in the normal manner. The non-returnvalve 55 also prevents the accumulator pressure from acting on thetractor hydraulic services.

A metering valve 160 (FIG. 1) is provided on the tractor for the purposeof hydraulically actuating the trailer brakes. The metering valve 160has an inlet port 61 connected by a branch line 62 to the accumulatorline 44, a brake port 63 connected by a line 64 to a tractor couplinghead 65 coupled to the coupling head 29, and a discharge port 66connected by a line 67 to the reservoir 15. The metering valve 160 alsohas a control port 68 connected by a line 69 to the master brakecylinder (not shown) of the tractor.

The metering valve 160 comprises a body 70 containing a stepped bore. Aportion 71 of this bore receives a piston 72 whose left hand face isexposed to a control chamber 73 connected to the control port 68. Thepiston 72 abuts a second piston 74 which is slidable in a bore portion75 of slightly larger diameter than the portion 71. The right hand faceof the piston 74 bounds a brake pressure cham ber 76 connected to thebrake port 63. A stem 77 on the piston 74 extends through the pressurechamber 76 and is guided in a reduced diameter portion 78 of the steppedbore. The right hand end of the reduced diameter bore portion 78provides a first valve seat 79 whilst the mouth of a longitudinalpassage 80 in the stem 77 provides a second valve seat 81 at the freeend of the stem 77. A conical valve closure member 82 cooperates withboth the valve seats 79 and 81. The closure member 82 has a stem 83which passes through a larger diameter portion 84 of the stepped boreand is sealingly guided in a reduced diameter portion 85 of this bore.The stem 83 Of the closure member 82 contains a longitudinal passage 86which leads from a chamber 87 at the right hand end of the stepped boreto oblique passages in the closure member 82. These oblique passageshave mouths so disposed in the conical face of the closure member 82that they communicate with the pressure chamber 76 whatever thepositions of the piston 74 and the closure member 82, i.e. they open ata diameter between the diameters of the valve seats 79 and 81. The boreportions 78 and 85 are of equal diameter so that the closure member 82is hydrostatically balanced. The larger diameter bore portion 84 definesa valve chamber 88 connected to the inlet port 61. The longitudinalpassage 80 in the piston stem 77 is connected by transverse bores to anannular groove 90 in the piston 74. This annular groove 90 communicateswith the mouth of a passage 91 leading to the discharge port 66 at leastwhen the valve seat 81 is separated from the valve closure member 82 asillustrated.

When the tractor brakes are not applied, the master cylinder does notdevelop any pressure so that there is no pressure in the control chamber73. The pistons 72 and 74 are thus biased to the left hand end positions(illustrated) by a spring 92 housed in the pressure chamber 76. Theclosure member 82 is biased against the seat 79 by a closure spring 93housed in the valve chamber 88 and it will be seen that in the extremeleftward position of the piston 74 the valve seat 81 is spaced from theclosure member 82 whereby the pressure chamber 76 is connected throughthe passage 80 the annular groove 90 and the passage 91 to the dischargeport 66 leading through the line 67 to the reservoir 15. The hydraulicactuators 21 and 22 are thereby free of pressure when the controlchamber 73 is not under pressure.

When the tractor brakes are applied, the master cylinder pressure istransmitted by the line 69 to the control chamber 73 and acts on thepiston 72. Since the pressure chamber 76 is under reservoir pressure(normally atmospheric pressure or some other very low pressure) thepistons 72 and 74 are urged to the right against the spring 92. When thevalve seat 81 engages the closure member 82 the pressure chamber 76 isisolated from the discharge port 66 and further movement of the pistons72 and 74 to the right lifts the closure member 82 from the valve seat79 thereby connecting the valve inlet chamber 88 to the pressure chamber76. The closure member 82 cooperating with the seat 79 meters the flowof hydraulic fluid from the accumulator chamber 175 (FIG. 2) via thelines 26, 44, 62, 64 and 28 to the hydraulic actuators 21 and 22. As thepressure in the pressure chamber 76 rises and approaches the pressure inthe control chamber 73 the pistons 74 and 72 are moved slightlyleftwards again to substantially shut off the flow between the closuremember 82 and the valve seat 79. Since the closure member 82 ishydrostatically balanced, the pressure in the pressure chamber 76 ismaintained by the metering valve proportional to the pressure in thecontrol chamber 73. The pressure applied to the hydraulic actuators 21and 22 is thereby directly proportional to the pressure developed by themaster cylinder. As the piston 74 is of only slightly larger diameterthan the piston 72, the pressure in the chamber 76 is maintained at avalue only slightly less than the pressure in the chamber 73. Ifdesired, the piston 74 could be made of slightly lesser diameter thanthe piston 72 or both pistons could be made of the same diameter. Thepistons 72 and '74 could be made integral with one another. When thebrake is released, the pressure in the control chamber 73 falls to zeroand the spring 92 urges the pistons 74 and 72 to their extreme leftwardpositions in which pressure chamber 76 is connected to the reservoir 15thereby releasing the pressure from the hydraulic actuators.

In the case wherein the tractor is fitted with purely mechanical brakesand has no master cylinder, a load cell can be fitted in one of thelinks of the tractor brakes. The load cell may be a hydraulic load cellconnected to the control chamber 73. Alternatively the load cell can beadapted to produce an electrical signal and for this purpose maycomprise an electrical strain gauge connected to an electrical circuitadapted to develop an electrical signal whose magnitude is proportionalto the applied brake force. In this case the metering valve 160 is asolenoid valve connected to receive this signal and adapted to adjustthe pressure in the brake pressure chamber to correspond to the signal.

When so much hydraulic fluid has flowed from the accumulator chamber 175(FIG. 2) that its pressure acting in the chamber 56 (FIG. 1) has fallento a value at which the spring 50 can close the closure member 49 of theunloader valve 18, the accumulator 170 is again charged by the pump 10in the previously described manner. Normally this lower pressure isstill higher than the minimum pressure at which the piston 177 is urgedaway from the stop 179 and the spring 36 of the accumulator is fullycompressed, as illustrated, with the abutment flange 94 on the piston172 abutting the body member 171 and the trailer brakes fully relievedof their mechanically applied force. However, should the pressure in thehydraulic accumulator fall substantially below this lower value, e.g.due to a fracture in the accumulator line 44, 26, the spring 36 urgesthe sleeve 37 and the piston 172 to the left thereby pulling on theconnecting rod 33 and the yoke 34 to apply the trailer brakesmechanically by the brake wires 35. Nevertheless suflicient hydraulicenergy is normally retained in the accumulator to permit several brakeactuations hydraulically, in the event of pump failure, before thebrakes are applied mechanically. The number of such hydraulic actuationsdepends upon the size of the accumulator.

A lever 161 (FIG. 1) is pivoted to the body 70 of the metering valve 160and acts on a plunger 162 which abuts the piston 72. The lever 161 canbe used for applying the trailer brakes independently of the tractorbrakes.

The dimensions of the hydraulic accumulator 170 (FIG. 2) including theforce of its spring 36 and the initial pressure of the gas cushion inthe chamber 176 are so chosen that the minimum pressure required in theaccumulator chamber to completely overcome the spring 36 isapproximately equal to the initial pressure in the chamber 176 at whichthe piston 177 rests against the stop 179. Thus during the initialcharging of the accumulator 170 through the line 26, the trailer brakesare mechanically relieved of the force of the spring 36 before thepiston 177 commences to move to the left.

Although the mechanical actuator and the hydraulic actuators are shownas acting on the same brakes it may be convenient to provide separatebrakes or at least separate brake shoes or pads for actuation by thehydraulic actuators and by the dual purpose accumulator.

A safety valve arrangement is fitted in the trailer lines 26 and 28.This valve arrangement has a housing 101 containing passages 102 and 103which connect the lines 26 and 28 to the coupling heads 27 and 29respectively. The housing 101 also has a medial longitudinal bore 104 inwhich a plunger 105 is slideable. Transverse bores 108 and 109 intersectthe passages 102 and 103. The inner ends of the transverse bores 108 and109 are tapered to provide valve seats for balls (not shown) springbiassed against these seats. The tapered inner ends of the bores 108 and109 break into the longitudinal bore 104 and the balls protrude intothis bore when they are seated. The plunger 105 has a shallow annulargroove (not shown) normally registering with the balls to permit them toseat. When the plunger 105 is moved forwardly by means yet to bedescribed the said balls are unseated and place the passages 102 and 103in communication with one another through the bores 108 and 109 and anannular groove 113 in the plunger 105.

The forward free end of the plunger 105 is coupled by a chain 180 on ablock 121 attached to the tractor vehicle.

The coupling heads 27, 29, 46 and 65 are of conventional constructionand can be separated by a firm pull without damage in the event that thetrailer breaks away from the tractor. Each of the coupling heads isprovided with a self-closing valve whose movable valve closure member127, 128 which is held open by abutment with the valve member 128, 127in the complementary head when the heads 27 and 29 are fitted to theircorresponding heads 46 and 65 and is automatically closed by springswhen the coupling heads are pulled apart.

The safety valve arrangement 100 and the coupling heads 27, 29, 46 and65 are described more fully in our co-pending patent application No.795,085 filed concurrently herewith.

Should the trailer break away from the tractor vehicle, the couplingheads 27 and 29 are pulled away from the heads 46 and 65 respectively.When the heads are separated, the valve springs urge the valve closuremembers 127 and 128 against their seats in the coupling heads to closeoff the coupling heads. At the same time the tractor pulls on theplunger 105 and the plunger 105 opens the ball valves to connect theaccumulator line 26 to the brake line 28. The hydraulic accumulator 170thereby automatically operates the trailer brake actuators 21 and 22 inthe event of a trailer break-away. Finally the chain 180 breaks topermit complete separation of the plunger 105 from the block 121.

The lines 26 and 28 or the lines 44 and 64 will normally include aflexible hose portion. Should the line 26 or the line 44 and inparticular the flexible hose portion thereof be fractured, there will bean immediate loss of pressure from the pressure accumulator 170 whichwill result in the spring 36 applying the trailer brakes mechanicallythereby warning the driver that a fracture has occurred. Should the line28 or the line 64 fracture then the driver may not notice the fractureuntil he applies the brakes. However, once he applies the brakes andpressure is developed in the control chamber 73 of the metering valve,the fracture prevents a corresponding pressure from being developed inthe brake pressure chamber 76. Consequently the closure member 82 isfully opened and the hydraulic fluid is immediately discharged from thehydraulic accumulator through the fracture and the consequent loss inpressure in the hydraulic accumulator permits the spring 36 to apply thetrailer brakes mechanically.

FIGS. 3 and 4 show another embodiment of the trailer brake system. InFIGS. 3 and 4, parts like those of FIGS. 1 and 2 are denoted by likereference numerals and will not be described again. FIG. 3 differs fromFIG. 1 principally in that the metering valve 60 is not provided withmeans 161, 162 for applying the trailer brakes independently of thetractor brakes. For simplicity the closure members in coupling heads 27,29, 46 and 65 are not shown in FIG. 3.

The hydraulic accumulator shown in FIG. 4 differs from the accumulator170 of FIG. 2 in that a cylindrical cavity 40 in the fixed body of theaccumulator 20 is divided by a free piston 43 into chambers 41 and 42 ofwhich the left hand chamber 41 is connected to the line 26 and serves tostore the hydraulic fluid whilst the right hand chamber 42 is chargedwith gas under a suitable pressure to provide a gas cushion. Thus apiston 31, which is slidable in the right hand end of the body 30 andwhich acts on the piston rod 32 and is abutted by the 8 sleeve 37, isexposed to the gas chamber 42 rather than the chamber 175 (FIG. 2) forthe hydraulic fluid. The gas cushion could be replaced by a springacting between the piston 43 and the piston 31.

The accumulator 20 is shown in FIG. 4 in its charged condition butinitially the piston 31 is in its leftward position and the piston 43 isin its extreme leftward position. When the pressure of the hydraulicfluid exceeds the pressure to which the gas cushion in the chamber 42 isinitially charged the piston 43 moves to the right and balances thepressures in the chambers 41 and 42. As the size of the chamber 41increases the gas cushion is further compressed and when the force ofthis pressure acting on the piston 31 becomes as large as the initialforce in the spring 36 the piston 31 is moved to the right therebysimultaneously relieving the trailer brakes of the mechanically appliedforce.

Normal operation of the braking system of FIGS. 3 and 4 is substantiallyas previously described with reference to FIGS. 1 and 2.

When so much hydraulic fluid has flowed from the accumulator chamber 41that its pressure acting in the chamber 56 (FIG. 3) has fallen to avalue at which the spring 50 can close the closure member 49 of theunloader valve 18, the accumulator 20 is again charged by the pump 10.Normally this lower pressure is still higher than the minimum pressureat which the spring 36 of the accumulator is fully compressed, asillustrated, with the abutment flange 94 on the piston 31 abutting thebody member 30 and the trailer brakes fully relieved of theirmechanically applied force. However, should the pressure in thehydraulic accumulator fall below this lower value, e.g. due to afracture in the accumulator line 44, 26, the spring 36 urges the sleeve37 and the piston 31 to the left thereby applying the trailer brakesmechanically. Initially, with the accumulator completely dis- 'charged,i.e. with the piston 43 fully to the left, the gas pressure in thechamber 42 acting on the piston 31 is insuflicient to overcome theinitial stress in the spring 36. When the chamber 41 has been filled toapproximately one third of its fully charged volume the pressure hasbeen sufficiently increased to completely overcome the spring 36 andcompletely relieve the trailer brakes of the mechanically appliedbraking force.

As an alternative, the force of the spring 50 of the unloader valve 18(FIG. 3) and the dimensions of the hydraulic accumulator 20 (FIG. 4)including the force of its spring 36 and the initial pressure of the gascushion in the chamber 42 may be so chosen in relation to the quantityof hydraulic fluid consumed by the actuators 21 and 22 upon a singleactuation of the brake by actuation of the master cylinder that thepressure in the accumulator falls below the pressure at which the spring36 begins to move the piston 31 to the left to apply the brakesmechanically. In this way the trailer brakes may be appliedsimultaneously mechanically and hydraulically.

The embodiment of the trailer brake system shown in FIGS. 5 and 6 issimilar to the embodiment of FIGS. 1 and 2 in a number of respects andthe like parts which are denoted by like reference numerals will not bedescribed again.

In, particular, the pump 10 for the basic tractor hydraulic services 11,12, the unloaded valve 18, the metering valve 160 and the trailer brakes21 to 24, 33 to 39 are the same.

However, a hydraulic accumulator 151 (FIG. 5) for the normal operationof the trailer brakes is mounted on the tractor. A dual purposehydraulic accumulator (FIG. 6) mounted on the trailer vehicle provides asource of hydraulic fluid under pressure for emergency operation of thetrailer brakes and is adapted to apply the trailer brakes mechanicallyin the event of a loss of pressure from the accumulator. That part ofthe brake system (FIG. 6) on the trailer is connected to the part of thesystem (FIG. 5) on the tractor by means of a quick-release self-sealingbreakaway coupling 141. The brake line 28 leads from the trailercoupling head 29 to the hydraulic actuators 21 and 22 and an accumulatorline 142 leads from the brake line 28 to the trailer-mounted accumulator140.

The hydraulic accumulator 140 (FIG. 6) comprises a fixed body 143 inwhich a piston 144 is slidable. The rod 145 of the piston 144 is coupledto the connecting rod 33 leading to the trailer brakes. A compressionspring 146 acts between the body 143 and the piston 144. Thus the spring146 urges the piston 144 to the left in a direction to apply the trailerbrakes mechanically and is sufficiently prestressed to effect fullapplication of the trailer brakes mechanically. The accumulator body 143contains a cylindrical cavity 147 which is divided into chambers 148 and149 by the piston 144. The chamber 148 serves to store hydraulic fluidunder pressure and is connected to the line 142 via a releasablenon-return valve 150. The chamber 149 accommodates the spring 146 and isvented to atmosphere. A gas cushion can replace or supplement the spring146 in the chamber 149.

The accumulator 151 (FIG. fitted to the tractor comprises a body 152containing a cylindrical cavity 153. The cavity 153 is divided intochambers 154 and 155 by a free piston 156. The chamber 154 serves tostore hydraulic fluid under pressure whilst the chamber 155 is chargedwith a gas under a suitable pressure to provide a gas cushion. The gascushion may be supplemented or replaced by a compression spring actionbetween the piston 156 and the end wall 157 of the body 152.

The accumulator chamber 154 is charged with hydraulic fluid through theunloader valve 18 and a line 158 leading from a pressure outlet port 45of the unloader valve 18 to the chamber 154.

The unloader valve 18 enables the pump 10, which may for example be agear pump and which normally operates under constant delivery quantitybut variable delivery pressure, to charge the accumulator chambers 148and 154 to a pre-determined pressure.

"Initially, when the pump is first started up and assuming that none ofthe basic tractor hydraulic services is operating, i.e. all the valvesare in their open-centre or open-neutral position, the fluid deliveredby the pump flows past the non-return valve 55 and through theaccumulator line 158 to the accumulator chamber 154. The accumulator 151is shown in FIG. 5 in its partially charged condition but initially thepiston 156 is in its extreme leftward position. When the pressure of thehydraulic fluid exceeds the pressure to which the gas cushion in thechamber 155 is initially charged the piston 156 moves to the right andbalances the pressure in the chambers 154 and 155. When the accumulatorpressure reaches its pre-determined value determined by the spring 50and the pressure in the chamber 53, the closure member 49 is moved tothe left thereby opening the valve to connect the inlet port 19 to thedischarge port 47 which is at reservoir pressure.

When the trailer is first hitched to the tractor, the accumulator 140(FIG. 6) is initially discharged so that the trailer brakes aremechanically applied by the spring 146. The accumulator is charged fromthe pump 10 by actuating the metering valve 160 (FIG. 5) and holdingthis valve fully open between the inlet port 61 and the brake port 63.The chamber 148 can thereby be charged to the pressure determined by theunloader valve 18 in exactly the same way as the chamber 154 of thetractormounted accumulator 151. The metering valve 160 can be held fullyopen either by holding the tractor brakes fully applied or bymanipulating the lever 161.

The non-return valve 150 (FIG. 6) is conveniently fitted to the body 142of the accumulator 140 and comprises a closure member 163 normally urgedby the pressure of the fluid in the chamber 148 against a seat 164. Aspring (not shown) may also act on the closure member 163 in thisdirection. A cam 165 is journalled in a bracket 166 on the body 142 andacts on the end of a valve stem 167 extending through the valve seat 164from the closure member 163. One end of a cord or chain 168 is attachedto the end of a lever 169 secured to the cam 165. The other end of thecord or chain 168 is hooked or otherwise attached to the tractor.

The coupling heads 29 and 65 of the quick-release, self-sealing,breakaway coupling 141 containnon-return valves whose respective closuremembers 127 and 128 are biassed towards their seats by closure springs129. When the head 29 is fitted to the head 65 the closure members 127and 128 abut one another so that the nonreturn valves are held open.

In the event of a trailer breakaway, the coupling heads 29 and 65 arepulled apart and their non-return valves are closed. At the same timethe pull on the cord or chain 168 opens the non-return valve 163 topermit the pressure in the accumulator to be applied to the actuators 21and 22 so that the trailer brakes are applied mechanically.

Normally before the trailer is uuhitched from the tractor, the lever 169is actuated to open the non-return valve and permit the hydraulic fluidto be discharged from the chamber 148 through the still-engaged coupling141 and the non-operated metering valve to the reservoir 15. This allowsthe spring 146 to pull on the connecting rod 33 and the yoke 34 to applythe trailer brakes mechanically by the brake wires 35.

If desired, the tractor mounted accumulator 151 can be used as a sourceof fluid for operating the tractor brakes. In this case the actuatorsfor the tractor brakes are connected to the brake line 64 from themetering valve and the brakes can be applied by manipulatlng the lever161, the control chamber 73 and control port 68 being omitted in thiscase.

In the case wherein the tractor vehicle is fitted with a hydraulicsystem of the constant pressure type, i.e. the type wherein the tractorhydraulic services are controlled by the so-called closed-centre" valvesand the pump 1s unloaded through a pressure controlling release valvewhen all the control valves are closed, the unloader valve 18 can bedispensed with and the pump 10 can be connected through a non-returnvalve directly to the line 44 (FIGS. 1 and 3) or 158 (FIG. 5).

As compared with the systems illustrated in FIGS. 1 to 4 the system ofFIGS. 5 and 6 has the advantages that there is only one pair of couplingheads and that the coupling heads are not required to hold pressureduring coupling and uncoupling (except in the event of a trailerbreakaway) and the disadvantage that the trailer brakes are not appliedautomatically in the event of a hose failure in the brake line 64, 28.

In another embodiment of the invention, a vehicle, which, fitted withthe pump 10, the accumulator 151 and the metering valve 160, is notprovided with a coupling head 65 but instead the brake line 64 isconnected directly to the brake actuators.

We claim:

1. A hydraulic system comprising a constant volume hydraulic pump havinga pump outlet, hydraulic services adapted to consume hydraulic power, atleast one opencentre type control valve having an inlet port, a serviceport and a drain port, said service port being connected to saidhydraulic services, a hydraulic accumulator adapted to store hydraulicfluid under pressure, and an unloader valve having an inlet port, apressure port, a drain port and a non-return valve connecting said inletport to said pressure port, said pressure port of said unloader valvebeing connected to said accumulator, said control valve and saidunloader valve being connected in series by their said inlet and drainports to form a series circuit, said series circuit being connected tosaid pump outlet, said control valve being adapted to control saidhydraulic services and said unloader valve further comprising a valveseat between said inlet port and said drain port, a valve closure membercooperating with said valve seat, a valve spring biassing said closuremember against said valve seat, an inlet chamber connected to said inletport, a pressure chamber connected to said pressure port, and

a piston connected to said valve closure member, said piston separatingsaid inlet chamber from said pressure chamber and being of largerdiameter than said valve seat, the fluid pressure in said pressurechamber acting on said piston in a direction to open said valve closuremember and the fluid pressure in said inlet chamber acting on said valveclosure member in said direction and upon said piston in an oppositedirection, whereby said valve closure member is opened to unload saidpump when the accumulator has been charged to a predetermined higherpressure and is closed to enable the pump to recharge the accumulatorwhen the accumulator pressure has fallen to a predetermined lowerpressure.

2. In a braking system for a vehicle having hydraulic brakes and basichydraulic services: a hydraulic accumulator to provide a source ofhydraulic fluid under pressure for operating said brakes, a hydraulicpump having a pump outlet, an unloader valve having an inlet port, apressure port, a drain port, a valve seat between said inlet port andsaid drain port and a valve closure member co operating with said valveseat, and movable away from said valve seat responsively to a firstpredetermined pressure applied to said pressure port and movable againstsaid valve seat to isolate said inlet port from said drain portresponsively to a second predetermined pressure at said pressure portlower than the first-mentioned predetermined pressure, at least oneopen-centre type control valve having at least one service port for saidbasic hydraulic services, said control valve having an inlet portconnected to said pump outlet and an outlet port connected to said inletport of said unloader valve, whereby said control valve and saidunloader valve are connected in series, means connecting said pressureport to said accumulator and means interconnecting said inlet port andsaid pressure port for charging said accumulator to said predeterminedpressure whereupon said valve closure member opens said valve seat toconnect said inlet port to said drain port to unload said pump.

3. A hydraulic system according to claim 1 in which said control valveand said unloader valve are connected in series with said drain port ofsaid control Valve connected to said inlet port of said unloader valve.

4. A hydraulic system according to claim 1 which further comprises ametering valve having an inlet port, a pressure port and a drain port,said inlet port of said metering valve being connected to saidaccumulator, said metering valve being adapted to controllably connectits said pressure port to its said inlet and drain ports.

5. A braking system according to claim 2 which further comprises ametering valve having an inlet port, a brake port and a drain port, saidinlet port of said metering valve being connected to said accumulator.

References Cited UNITED STATES PATENTS 2,264,375 12/1941 Hill et a1.60-51 X 2,707,002 4/ 1955 Harris. 2,793,498 5/1957 Banker 60-513,133,417 5/1964 Hager 60-51 3,205,659 9/1965 Hartzell 6051 EDGAR W.GEOGHEGAN, Primary Examiner US. Cl. X.R. 303-7

